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      mysql架构简介
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	<a href="/2021/01/26/mysql架构简介/" class="article-date">
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        <h2 id="整体架构图"><a href="#整体架构图" class="headerlink" title="整体架构图"></a>整体架构图</h2><img src="https://jameslin23.gitee.io/2021/01/26/mysql架构简介/QQ图片202101261400401png.png" style="zoom:80%;">

<p>　　和其它数据库相比，MySQL 有点与众不同，它的架构可以在多种不同场景中应用并发挥良好作用。主要体现在<br>存储引擎的架构上，<strong>插件式的存储引擎架构将查询处理和其它的系统任务以及数据的存储提取相分离</strong>。这种架构可<br>以根据业务的需求和实际需要选择合适的存储引擎。</p>
<h3 id="连接层"><a href="#连接层" class="headerlink" title="连接层"></a>连接层</h3><p>　最上面一层，用于一些客户端和连接服务，如jdbc,odbc等。包含了本地sock通信和大多数基于客户端/服务端工具实现的类似与tcp/ip的通信，主要完成一些连接处理、授权认证、及相关的安全方案。在该层引入线程池的概念。为通过认证安全接入的客户提供线程。同样该层上可以实现基于SSL的安全链接。服务器也会为安全接入的每个客户端验证它所具有的操作权限。</p>
<h3 id="服务层"><a href="#服务层" class="headerlink" title="服务层"></a>服务层</h3><h4 id="Management-Serveices-amp-Utilities"><a href="#Management-Serveices-amp-Utilities" class="headerlink" title="Management Serveices &amp; Utilities"></a>Management Serveices &amp; Utilities</h4><p>​      系统管理和控制工具。</p>
<h4 id="SQL-Interface"><a href="#SQL-Interface" class="headerlink" title="SQL Interface"></a>SQL Interface</h4><p>​       SQL接口，接受用户SQL命令、并且返回用户需要查询的结果。比如select from</p>
<h4 id="Parser"><a href="#Parser" class="headerlink" title="Parser"></a>Parser</h4><p>​       解析器，SQL命令传递到解析器的时候会被解析器验证和解析。</p>
<h4 id="Optimizer"><a href="#Optimizer" class="headerlink" title="Optimizer"></a>Optimizer</h4><p>​       查询优化器。SQL语句在查询之前会使用查询优化器对查询进行优化，比如where条件时，优化器来决定先投影还是先过滤。</p>
<h4 id="Cache和Buffer"><a href="#Cache和Buffer" class="headerlink" title="Cache和Buffer"></a>Cache和Buffer</h4><p>​       查询缓存。如果查询缓存有命中的查询结果，查询语句就可以查询缓存中取数据，这个缓存机制是由一系列小缓存组成，比如表缓存，记录缓存，记录缓存，key缓存，权限缓存。</p>
<h3 id="引擎层"><a href="#引擎层" class="headerlink" title="引擎层"></a>引擎层</h3><p>​       此层真正负责了Mysql中数据的存储和提取，服务器通过API与存储引擎进行通信，不同存储引擎具有的功能不同，这样我们可以通过自己业务需求进行选取。</p>
<h3 id="存储层"><a href="#存储层" class="headerlink" title="存储层"></a>存储层</h3><p>​     数据存储层，主要是将数据存储在运行于裸设备的文件系统之上，并完成与存储引擎的交互。</p>
<h2 id="profile"><a href="#profile" class="headerlink" title="profile"></a>profile</h2><p>利用show profile可以看到sql的执行周期</p>
<p><strong>开启 profile</strong>：set profiling = 1开启</p>
<p><strong>查看是否开启</strong>：show variables like ‘%profiling%’</p>
<img src="https://jameslin23.gitee.io/2021/01/26/mysql架构简介/image-20210126144056654.png" alt="image-20210126144056654" style="zoom:67%;">

<p><strong>使用profile</strong>：show profilies 命令</p>
<img src="https://jameslin23.gitee.io/2021/01/26/mysql架构简介/image-20210126144206917.png" alt="image-20210126144206917" style="zoom:67%;">

<p>根据Query_ID，可以进一步执行 show profile cpu,block io for query Query_id 来查看sql的具体执行步骤。</p>
<img src="https://jameslin23.gitee.io/2021/01/26/mysql架构简介/image-20210126144333551.png" alt="image-20210126144333551" style="zoom:67%;">

<p><strong>大致查询流程</strong></p>
<p>​        mysql客户端通过协议与mysql服务器连接，发送查询语句，先检查查询缓存，如果命中，直接返回结果，否则进行语句解析，就是说，再解析查询之前，服务器会先访问查询缓存（query cache）—它存储SELECT语句以及相应查询结果集。如果某个查询结果已经位于缓存中，服务器不会再对查询进行解析、优化、以及执行。它仅仅将缓存中的结果返回给用户即可，大大提升系统的性能。</p>
<p>​      语法解析器和预处理：首先mysql通过关键字将SQL语句进行解析，并生成一颗相应的“解析树”。mysql解析器将使用mysql语法规则进行验证和解析查询；预处理则根据一些mysql规则进一步检查和解析数是否合法。</p>
<p>​       查询优化器当解析数被认为是合法的，并且由优化器将其转化成执行计划。一条查询可以有很多种执行方式。最后都返回相同的结果。优化器的作用就是找到其中最好的执行计划。</p>
<p><strong>我们手写SQL语句顺序</strong></p>
<img src="https://jameslin23.gitee.io/2021/01/26/mysql架构简介/image-20210126145957348.png" alt="image-20210126145957348" style="zoom:80%;">

<p>MYSQL可能调整执行顺序</p>
<img src="https://jameslin23.gitee.io/2021/01/26/mysql架构简介/image-20210126150149038.png" alt="image-20210126150149038" style="zoom: 80%;">

<p>随着mysql迭代更新，其优化器也不断的升级，优化器会分析不同执行顺序产生的性能优化消耗不同而动态调整执行顺序。</p>

      
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        <ol class="nav"><li class="nav-item nav-level-2"><a class="nav-link" href="#整体架构图"><span class="nav-number">1.</span> <span class="nav-text">整体架构图</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#连接层"><span class="nav-number">1.1.</span> <span class="nav-text">连接层</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#服务层"><span class="nav-number">1.2.</span> <span class="nav-text">服务层</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#Management-Serveices-amp-Utilities"><span class="nav-number">1.2.1.</span> <span class="nav-text">Management Serveices &amp; Utilities</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#SQL-Interface"><span class="nav-number">1.2.2.</span> <span class="nav-text">SQL Interface</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#Parser"><span class="nav-number">1.2.3.</span> <span class="nav-text">Parser</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#Optimizer"><span class="nav-number">1.2.4.</span> <span class="nav-text">Optimizer</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#Cache和Buffer"><span class="nav-number">1.2.5.</span> <span class="nav-text">Cache和Buffer</span></a></li></ol></li><li class="nav-item nav-level-3"><a class="nav-link" href="#引擎层"><span class="nav-number">1.3.</span> <span class="nav-text">引擎层</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#存储层"><span class="nav-number">1.4.</span> <span class="nav-text">存储层</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#profile"><span class="nav-number">2.</span> <span class="nav-text">profile</span></a></li></ol>
    
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